arm: imx8m: remove unused and obsolete board_fix_fdt() in SOC context

arm: imx8m: remove unused and obsolete board_fix_fdt() in SOC context

It doesn't seem appropriate for arch/SOC to use a board-level
functionality (CONFIG_OF_BOARD_FIXUP), because this prevents boards
that need to do FDT fixup from using that feature.

Also, this code is completely dead and useless (from comments by
Rasmus Villemoes on the mailing list):

  - No in-tree imx8m-based board seems to set CONFIG_OF_BOARD_FIXUP
  - The nodes which that function wants to disable don't even exist in
    the U-Boot copy of imx8mp.dtsi.

This code was introduced in commit 35bb60787b88. It seems to be some
random import of code from downstream NXP U-Boot, with a commit
message that makes no sense in upstream context.

Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Acked-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>